FIG. 1 shows a driver circuit according to the prior art for driving a useful signal.
The driver circuit illustrated in FIG. 1 is of differential construction and contains two operational amplifiers OPA, OPB for signal amplification. The operational amplifiers amplify a useful signal arriving from a signal source and output the amplified useful signal via protection impedances Za, Zb to a connected terminal, for example a telephone T. The operational amplifiers are integrated for example in an SLIC circuit situated on a line card. The operational amplifiers each have a low output impedance for the signal amplification of the useful signal. The impedances connected downstream of the two operational amplifiers serve to protect the amplifier circuits and for electromagnetic compatibility (EMC). The output impedances Za, Zb preferably protect the amplifier circuit from overvoltages, which may be caused by a flash of lightning, for example, and for the suppression of interference signals, for example radio signals.
FIGS. 2a to 2c show practical realizations of the protection impedances Za, Zb according to the prior art.
The driver circuit illustrated in FIG. 1 is of differential construction, the components being symmetrical, i.e. in particular the two protection impedances Za, Zb are as far as possible of identical construction in order to have a maximum longitudinal conversion loss. In communication systems, for example appertaining to voice telephony, the driver circuit for driving the useful signal must satisfy very stringent circuitry requirements with regard to the longitudinal conversion loss LCL. The standards relevant to the longitudinal conversion loss are the TR57 standard in the USA and the Q552 and G712 standards in Europe.
FIG. 3 shows a measuring circuit for determining the longitudinal conversion loss LCL.
FIG. 4 shows an associated equivalent circuit diagram. The output impedance Zout illustrated in the equivalent circuit diagram is the output impedance of the overall circuit to the left of the output pads Outa, Outb for the SLIC circuit in FIG. 1. The measuring circuit illustrated in FIG. 3 serves for measuring the longitudinal conversion loss LCL. A signal source feeds a sinusoidal measurement signal into the two output pads A, B of the line card via feed-in resistors RL. The two measuring resistors RL are high-precision resistors with matching resistances. The voltage between the two output pads A, B is measured.
The following holds true for the longitudinal conversion loss:                     LCL        =                              20            ·            log                    ⁢                                                                V                L                                            V                T                                                                                    (        1        )            The following relationship holds true between the longitudinal conversion loss LCL and the impedances illustrated in the equivalent circuit diagram according to FIG. 4:                     LCL        =                              20            ·            log                    ⁢                                                1                                                                                          Z                      out                                        +                                          Δ                      ⁢                                                                                          ⁢                      Z                                                                                                  R                      L                                        +                                          Z                      out                                        +                                          Δ                      ⁢                                                                                          ⁢                      Z                                                                      -                                                      Z                    out                                                                              R                      L                                        +                                          Z                      out                                                                                                                                          (        2        )            The high-precision feed-in resistors RL have a value of 300 ohms, for example. The impedance difference ΔZ between the output impedances exists because of manufacturing tolerances and inaccuracies of the protection impedances Za, Zb in the conventional driver circuit as illustrated in FIG. 1.
With the applicable condition ΔZ<<RL, equation (2) can be greatly simplified to:                     LCL        ≅                              20            ·            log                    ⁢                                                                                  R                  L                                +                                  Z                  out                                                            Δ                ⁢                                                                  ⁢                Z                                                                                    (        3        )            
To ensure that the longitudinal conversion loss is as high as possible and LCL thus assumes a maximum value, conventional driver circuits have hitherto used expensive output impedances with high accuracy, i.e. impedances which have very small tolerances during production. Such components which have to satisfy very high accuracy requirements can only be produced with considerable technical outlay and high costs.